Conventional known ways of charging a rechargeable battery include CCCV (constant current-constant voltage) charging schemes wherein constant current charging, in which charging is carried out at a constant current value, is performed initially, and when the terminal voltage of the rechargeable battery reaches a charge termination voltage set beforehand, there is performed constant voltage charging, in which the battery is charged at a constant charge voltage, through application of the charge termination voltage to the rechargeable battery (for instance, see Patent document 1).
In a CCCV charging scheme, the open-circuit voltage (OCV) at the time of full charge when the SOC (State of Charge) of the rechargeable battery is 100%, i.e. the full charge voltage, is set as the charge termination voltage. The rechargeable battery has an internal resistance R. Therefore, when the terminal voltage of the rechargeable battery reaches the charge termination voltage by constant current charging, the terminal voltage of the rechargeable battery includes a voltage drop IR derived from the charge current I flowing through the internal resistance R. Consequently, the open-circuit voltage of the rechargeable battery falls still short of the charge termination voltage (=full charge voltage), and, accordingly, the rechargeable battery is not yet in full charge.
Thus, further constant voltage charging is accompanied by a gradual decrease in the charge current and in a decreased voltage drop IR. The open-circuit voltage of the rechargeable battery increases in proportion to the reduction in voltage drop IR. The rechargeable battery can be brought to full charge by terminating charging when the charge current is equal to or smaller than a charge termination current value, set beforehand to a small current value, and the voltage drop IR becomes negligibly small, i.e. when the open-circuit voltage of the rechargeable battery becomes substantially identical to the full charge voltage.
In such a CCCV charging scheme, the charging time is shortened by charging the rechargeable battery at a constant current value that is comparatively large, for instance about 0.7 It, until the close-circuit terminal voltage of the rechargeable battery reaches the full charge voltage. Herein, 1 It (battery capacity (Ah)/1 (h)) denotes the current value at the point in time where the residual charge of the rechargeable battery becomes zero after one hour, for constant-current discharge of the nominal capacity value of the rechargeable battery.
In a CCCV charging scheme, once the close-circuit terminal voltage of the rechargeable battery reaches the full charge voltage, the charge current decreases naturally through constant voltage charging at the full charge voltage. Therefore, deterioration of the rechargeable battery through overcharging is to be avoided.
When in a CCCV charging scheme constant current charging is carried out in a state where the closed-circuit terminal voltage of the rechargeable battery has not reached yet the full charge voltage, however, deterioration due to flow of charge current is likelier to occur in a state where the closed-circuit terminal voltage has reached the vicinity of the full charge voltage, through an increase in the SOC of the rechargeable battery, than at a time when the SOC of the rechargeable battery is small. This is problematic in that, as a result, the rechargeable battery may deteriorate when constant current charging is carried out, at a same current value, from a state where SOC is close to 0% up to a state close to full charge. The above phenomenon was particularly marked at low temperature and high temperature.